An organic EL display panel refers to a display panel which includes light-emitting devices exploiting electroluminescence (EL) of organic compounds. Specifically, the organic EL display panel has EL devices each including a pixel electrode, an organic luminescent layer disposed over the pixel electrode, and a counter electrode disposed over the organic luminescent layer. Organic EL materials used for the organic luminescent layer can be broadly classified into two types: combinations of low-molecular weight compounds (combinations of host and dopant materials); and organic polymer compounds. Examples of organic polymer compounds include polyphenylenevinylene (abbreviated as “PPV”) and its derivatives. An organic EL display panel that utilizes organic polymer compounds can be driven at relatively low voltage and consumes less power, lending itself to development of large display panels. Under this circumstance, extensive research activities are underway.
With a printing technology such as inkjet printing, organic polymer compounds are applied to corresponding pixels according to their luminescence color—R, G or B. For example, printing is accomplished by discharging a polymer ink containing an organic polymer compound and solvent from an inkjet head. When printing such a polymer ink on certain pixels, it is necessary to prevent the polymer ink from entering unintendedly into adjacent pixels.
In order to prevent an organic EL material-containing ink from entering into adjacent pixels of different colors, a method is suggested in which walls (banks) are provided in such a way as to surround all four sides of every pixel and the ink is dropped in regions defined by the banks (see, e.g., Patent Document 1).
Nevertheless, organic EL display panels manufactured with this method have had the disadvantage of non-uniform organic luminescent layer thickness because discrete organic luminescent layers are formed in regions defined by banks. Namely, non-uniform thickness is caused by the surface tension of the applied ink, which pulls more liquid to the edges of the banks. Non-uniformity in organic luminescent layer thickness reduces the luminescence efficiency and, accordingly, lifetime of an organic EL display panel.
FIG. 1A shows an organic EL display panel structure which has succeeded in overcoming the above disadvantage (see, e.g., Patent Document 2). FIG. 1A is a plan view of the organic EL display panel disclosed by Patent Document 2. FIG. 1B is a III-III line sectional view of the organic EL display panel shown in FIG. 1A. FIG. 1C is a partially enlarged view of FIG. 1B. As shown in FIGS. 1A to 1C, the organic EL display panel includes glass substrate 1, first electrode layer 2, first banks 4, second banks 3, hole injection layers 5, and organic luminescence layers 6. First bank 1 and second bank 3 are both made of resin.
First banks 1 define linear regions 40. Each linear region 40 has a row of organic EL devices and defines linear organic luminescence layer 6. Second banks 3 define pixel regions 30, which define regions of hole injection layer 5.
In the organic EL display panel shown in FIGS. 1A to 1C each organic luminescent layer covers multiple pixel regions (organic EL devices), whereby the thickness of each organic luminescent layer can be made uniform in lengthwise direction. To achieve this configuration, organic luminescent layers 6 are formed so as to also cover second banks 3.
A method is known, to control the “pinning” of functional layers (hole injection layer and organic luminescent layer) by employing a two-layered resin bank defining the functional layers obtained by coating method (see, e.g., Patent Document 3). In the organic EL device disclosed by Patent Document 3, a first resin layer of a bank (lower bank layer) is made smaller in width than a second resin layer (upper bank layer) to provide a difference in bank height. This bank level difference controls “pinning” of the functional layers to provide uniform thick functional layers.
Moreover, methods are known in which a two-layered bank is employed whose upper bank is made lyophobic and whose lower bank is made lyophilic, so that functional layer material solution can sufficiently spread over the region defined by the bank (see Patent Documents 4-10). In the organic EL devices disclosed by Patent Documents 4-10, the lower bank is made lyophilic by employing a lyophilic material such as SiO2 as a lower bank material. This increases compatibility between the bank and functional layer material solution, allowing the applied functional layer material solution to spread over the entire region defined by the bank.
Patent Document 1: Japanese Patent Application Publication No. 2006-86128
Patent Document 2: U.S. Pat. No. 7,091,660
Patent Document 3: Japanese Patent Application Publication No. 2006-41027
Patent Document 4: Japanese Patent Application Publication No. 2006-286309
Patent Document 5: Japanese Patent Application Publication No. 2006-305331
Patent Document 6: Japanese Patent Application Publication No. 2006-71872
Patent Document 7: Japanese Patent Application Publication No. H11-329741
Patent Document 8: Japanese Patent Application Publication No. 2007-44582
Patent Document 9: Japanese Patent Application Publication No. 2005-222776
Patent Document 10: Japanese Patent Application Publication No. 2006-294446